In 1965 she began working as part of a team lead by Alexander Thomas, (including Stella Chess, Margaret Hertzig and Sam Korn) on a longitudinal study to follow a sample of over a hundred children from infancy to adulthood. Thomas and his colleagues identified nine characteristics of temperament that were stable and endured through maturity. They include: activity level, regularity (of biological functions), adaptability, threshold of responsiveness, intensity of reaction, mood quality, distractibility, persistence, and attention span. From these nine characteristics, three major patterns emerged that accounted for 65% of the children: the easy child, accounting for 40% of the sample, the difficult child, account for 10% of the sample, and the slow-to-warm-up child, which accounted for 15% of the sample. The remaining 35% showed no consistent temperament patterns.